Ultrastructural and cytochemical studies of myocardial biopsies disclosed the presence of glycogen deposits within nuclei of hypertrophied and normal-sized cardiac muscle cells in 6 (7%) of 90 patients with various cardiac diseases. Intranuclear glycogen appeared as beta-particles, 160 to 360 A in diameter, which either formed small agregates or were dispersed in the nucleoplasm. Cells containing intranuclear glycogen did not show damage or degeneration. It is concluded that under certain conditions the nuclei of cardiac muscle cells can acquire the capacity to synthesize glycogen.